Permafrost or perennially frozen ground is an important part of the terrestrial cryosphere; roughly one quarter of Earth s land surface is underlain by permafrost. The impact of the currently observed warming, which is projected to persist during the coming decades due to anthropogenic CO2 input, certainly has effects for the vast permafrost areas of the high northern latitudes. The quantification of these effects, however, is scientifically still an open question. This is partly due to the complexity of the system, where several feedbacks are interacting between land and atmosphere, sometimes counterbalancing each other. Moreover, until recently, many global circulation models (GCMs) and Earth system models (ESMs) lacked the sufficient representation of cold region physical soil processes in their land surface schemes, especially of the effects of freezing and thawing of soil water for both energy and water cycles. Therefore, it will be analysed in the present study how these processes impact large-scale hydrology and climate over northern hemisphere high latitude land areas. For this analysis, the atmosphere-land part of MPI-ESM, ECHAM6-JSBACH, is driven by prescribed observed SST and sea ice in an AMIP2-type setup with and without newly implemented cold region soil processes. Results show a large improvement in the simulated discharge. On one hand this is related to an improved snowmelt peak of runoff due to frozen soil in spring. On the other hand a subsequent reduction of soil moisture leads to a positive land atmosphere feedback to precipitation over the high latitudes, which reduces the modelâs wet biases in precipitation and evapotranspiration during the summer. This is noteworthy as soil moisture - atmosphere feedbacks have previously not been in the research focus over the high latitudes. These results point out the importance of high latitude physical processes at the land surface for the regional climate.
2.2 Experimental setup
Two ECHAM6.3/JSBACH simulations were conducted at T63 horizontal resolution (about 200 km) with 47 vertical layers in the atmosphere. They were forced by observed sea surface temperature (SST) and sea ice from the AMIP2 (Atmospheric Model Intercomparison Project 2) dataset for 1970-2009 (Taylor et al., 2000). 1970-1988 are regarded as spin-up phase so that only the period 1989-2009 is considered for the analyses and stored in the archive. The two simulations are:
- ECH6-REF: Simulation with the standard version of JSBACH 3.0 with a fixed vegetation distribution and using a separate upper layer reservoir for bare soil evaporation as described in Hagemann and Stacke (2015). Note that the latter is switched off by default in JSBACH 3.0 to achieve a better performance of simulated primary productivity, which is not of interest in the present study.
- ECH6-PF: As ECH6-REF, but using JSBACH-PF.
Note that both simulations used initial values of soil moisture, soil temperature and snowpack that were obtained from an offline-simulation (land only) using JSBACH (as in ECH6-REF) forced with WFDEI data (Weedon et al., 2014).
As carbon cycle and vegetation dynamics were not considered, and the focus of the study was on the land surface, only monthly mean data from the ECHAM BOT stream as well as from the JSBACH main and land streams are stored together with river discharges and the JSBACH initial files.